1. Field of the Invention
This invention relates to catalysts. More specifically, it relates to solid-phase heterogeneous catalysts, especially metal surface catalysts used to enhance the rate of gas-phase reactions.
2. Description of the Problem
Catalysts are defined as substances which when added to a reaction mixture in less than stoichiometric quantities effect a change in the rate of the reaction. Catalysts are almost always employed to enhance the rate of reactions by lowering the energy of activation from that which prevails in the absence of a catalyst--i.e., the catalyst changes the mechanistic pathway. A catalyst affects the rate of a chemical reaction without itself being consumed or undergoing a chemical change. Thus, a catalyst can (theoretically) last indefinitely and/or be quantitatively recovered.
In practice, catalysts do not last indefinitely. The life of an industrial catalyst commonly varies from 1000 to 10,000 hours, after which it must be replaced or regenerated. This is especially true of solid-phase catalysts since the activity of a solid catalyst is often centered in a small fraction of its surface, and any decrease in the number of active points can significantly affect the activity of the catalyst. Substances which block or otherwise alter the catalytic surface in a way which decreases its activity are called "poisons." An oft-cited example of a catalyst poison is lead. Organic lead compounds (e.g., tetraethyllead, the antiknock compound used in some motor vehicle fuels) are known to virtually destroy the catalytic activity of emission control devices employing precious-metal catalysts such as platinum and palladium.
Of particular interest in the present context is a type of catalyst poisoning known as "coking." Coking occurs on the surface of a metal catalyst via the formation of metal-carbon bonds. This phenomenon has been observed in the nickel-catalyzed hydrogenolysis of C.sub.2 H.sub.6 to CH.sub.4. This particular reaction has been the subject of numerous kinetic studies. The reaction proceeds at a measurable rate at temperatures as low as 200.degree. C. The observed rate increases rapidly as a function of temperature up to approximately 250.degree. C.; and, less rapidly from about 250.degree. to 350.degree. C. Above 350.degree. C. the reaction rate is relatively temperature independent. At temperatures between approximately 500.degree. and 700.degree. C. a decrease in the reaction rate is observed (relative to those measured at around 350.degree. C.).
The temperature dependence of this reaction, including the slower reaction rates observed above about 500.degree. C., has been the subject of numerous studies. These studies indicate that the decrease in the rate of the reaction at high temperatures is related to the formation of Ni--C bonds on the surface of the catalyst. This high temperature "coking" effects a poisoning of the catalyst with respect to the hydrogenolysis reaction. Indeed, recent surface studies have produced not only molecular confirmation of coking, but actual evidence of specific molecular structures for the surface-formed molecules. For example, cross-linking of initially formed unsaturated carbon chains which results in the eventual formation of a two dimensional graphite-like deposit is believed to be responsible for the high temperature inhibition of nickel catalysis in hydrogenolysis and dehydrogenolysis reactions. Other studies indicate that the formation of multiple metal-carbon atom bonds plays an important role in this inactivation process.
Regardless of the exact nature of the carbon-nickel molecular formations, the important fact from a practical standpoint is that those molecular interactions which lead ultimately to the deactivation of the nickel catalyst are significant at temperatures as low as 250.degree. C. and become progressively more important with increasing temperatures. Regeneration of catalytic activity can be achieved with fair success at temperatures below about 250.degree. C. by the use of hydrogen or steam treatments. At higher temperatures, the direct in situ regeneration of catalytic activity is much more difficult to achieve. It appears that the high temperature coking of the nickel catalyst results in the formation of metal carbide bonds and this process is virtually irreversible in terms of normal chemical regeneration techniques. It has been noted that treatment with oxygen can remove carbon formations at lower temperatures, but metal-oxide formation results at higher temperatures which decreases catalytic activity. Oxygen is apparently not successful in catalytic regeneration at temperatures above 360.degree. C. where graphite formation is observed.
Nickel is used as a catalyst in many important industrial processes. For example, the steam-hydrocarbon reforming process: ##EQU1## This reaction is commonly carried out on C.sub.1 to C.sub.4 hydrocarbons at high temperatures (760.degree. to 980.degree. C.) in the presence of a Ni catalyst. The products are usually subsequently subjected to a shift reaction at between 315.degree. and 370.degree. C. in order to maximize the production of hydrogen (CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2). Poisoning of the nickel catalyst used in the reforming process is a serious economic problem.
Methanation reactions (e.g., CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O) are another example of nickel catalyzed reactions in which catalytic poisoning is a major problem. These reactions are commonly carried out at temperatures of about 315.degree. C.
A major industrial use of Ni involves the hydrogenation of fats and oils. Hydrogenation is used to improve the keeping qualities, taste, and odor of such products. A typical example would be: EQU (C.sub.17 H.sub.31 COO).sub.3 C.sub.2 H.sub.5 +3H.sub.2 .fwdarw.(C.sub.17 H.sub.33 COO).sub.3 C.sub.3 H.sub.5
The nickel catalyst employed in these hydrogenations is easily inactivated, especially by sulfur-containing compounds such as sulfur dioxide and hydrogen sulfide.
The catalytic cracking of ammonia is another important process which employs a nickel catalyst: ##STR1## The reaction is carried out at 870.degree. C. It is sometimes used for the on-site production of hydrogen inasmuch as ammonia is easier and less hazardous to transport than hydrogen. Again, the nickel catalyst is subject to poisoning.
A number of other metal catalysts used in industrially important processes are subject to poisoning and/or deactivation. Precious metals such as platinum, palladium, rhodium, ruthenium, osmium, and iridium are used in a wide variety of hydrogenations including the conversion of olefins to paraffins, dienes to monoenes, benzene to cyclohexane, alkynes to alkenes and/or paraffins, aldehydes to alcohols, nitro compounds to organo amines, resorcinols to dihydroresorcinols, ketones to alcohols, and cyanides to amines. In general, the catalysts employed are prone to deactivation via the deposition of surface inhibitors, particularly from sulfur-containing compounds.
The synthesis of ammonia from nitrogen employs an iron catalyst which contains a small quantity of mixed oxides: EQU N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3
The catalyst in this process is also subject to inactivation, especially at higher temperatures.
The 3-way catalytic converters used on automobiles in this country are another example of catalysts which suffer from surface deactivation by poisoning. These converters are used to oxidize carbon monoxide and unburned hydrocarbons and to reduce nitric oxide. Deposits on the catalytic surfaces, particularly those from lead and sulfur compounds, deactivate the catalysts and disable the converter.